The present invention relates to a mobile communication apparatus and method, or more in particular to a mobile radio communication apparatus employing a digital modulation scheme requiring a comparatively wide range of output power and the linearity of a radio-frequency amplification section. The invention specifically relates, for example, to a mobile communication technique effectively applicable to automobile telephones and portable radio telephones (hereinafter referred to simply as "handy telephone" or "the portable phone") used in a cellular telephone system.
The portable phone is based on the technique for restricting the transmission power in such a manner that the radio wave is prevented from propagating to an excessively far point and for using the same frequency at a far point, i.e., in other cells in order to permit servicing as many subscribers as possible within the finite frequency resources allocated to the system. In this portable phone, the magnitude of transmission output power is determined by the distance and the electrical communication environment between the transmitter and the receiver. More specifically, explanation will be made with reference to an example of the cellular telephone system emphasizing a given base station as shown in FIG. 10. In the communications between the base station 103 and a portable phone set 101 similar to each other in the electrical communication environment, the transmission power is reduced as needed. In the case of a portable phone set 102 which is located at a long distance from the base station 103, on the other hand, the communication power between the base station 103 and the portable phone 102 is increased. As a result, the control of the transmission output power within the range of about 20 dBm, for example, is required for the transmitters 101, 102. The level of transmission power used by the transmitters 101, 102 for transmission is selected by the signal, i.e., the instruction on the transmission power information contained in the received signal sent from the base station. This control of transmission power is intended to minimize the effect of unnecessarily large transmission power on the devices used for communications in other cells with a relatively short distance between the transmitter and the receiver, to minimize the consumption of the power battery of the transmitter due to unduly large transmission power and to thereby lengthen the communication time with a limited battery capacity. This control of transmission power is described in JP-A-3-229526 laid open Oct. 11, 1991 and JP-A-4-37323 laid open Feb. 7, 1992.
The above-mentioned control of transmission power can be easily realized by controlling the gain of the radio-frequency power amplifier of the transmitter. The inventors, however, have discovered that the mere gain control of the output power makes it difficult to attain compatibility between the linearity of the amplifier during a small output power and that during a large output power due to the fact that the input signal power of the radio-frequency power amplifier is fixed, and therefore the modulation output signal is liable to interfere with adjacent channels.
In other words, in cellular telephone systems, it is necessary to prevent interference with adjacent channels by narrowing the bandwidth of a given transmission signal (a given modulation signal). For this purpose, the format of the frequency spectrum representing the distribution of the signal power of each frequency and the frequency of the side lobes about the carrier frequency is specified as shown in FIG. 11A. The transmission signal is required not to exceed the upper-limit power level specified by this format. A modulation system convenient for reducing the interference with adjacent channels includes the GMSK (Gaussian Minimum Shift Keying) or the .pi./4-shift QPSK. Further, in order to satisfy the specification of the frequency spectrum, the radio-frequency power amplifier is required to have a linearity of output power with respect to input power within the operating power range.
It is, however, difficult to secure sufficient linearity over the whole range of operating power due to the non-linear characteristic of the semiconductor devices during small power output. As shown in FIG. 12, for example, when the gate-source voltage Vgs is comparatively small, the-relation between the gate-source voltage Vgs and the drain current Id of a MOS transistor is extremely nonlinear (class-B amplification). Assume that the gain of the radio-frequency power amplifier is to be controlled by controlling the gate bias voltage of the MOS transistor. When an attempt is made to produce a small power by reducing the gate bias voltage of the MOS transistor, the output signal waveform is distorted with the arrival of an input signal to the particular gate. This distortion displaces the transmission signal from the frequency spectrum specification.
An amplitude-modulation output in percent making up one of the parameters for evaluating the amplification linearity is shown in FIG. 13. The characteristic shown in FIG. 13 is obtained by applying a 915-MHz transmission wave signal subjected to 10-KHz 5% amplitude modulation to a radio-frequency power amplifier made up of a silicon MOS transistor, producing the desired gain of the output power by regulating the voltage at the gain control terminal of the particular amplifier, and measuring the proportion of the 10-KHz amplitude modulation component contained in the output power, as shown in FIG. 14. Compare, for example, the class-B amplification with a reduced gate bias voltage with the class-A amplification with an increased gate bias voltage (FIG. 12). With the class-A amplification, the drain current flows over the whole period of input, and therefore, the modulation component occupying the amplified output amplitude is reduced, i.e., the distortion of the amplified modulation signal is reduced as compared with the class-B amplification. This is clearly indicative of the fact that the distortion of the modulation signal is increased with the value of the amplitude modulation output in percent. The inventors discovered that a large amplitude modulation output in percent is equivalent to the existence of many frequency components displaced from the frequency spectrum specification. This is clearly indicated by the trend shown in FIG. 11B. The larger the amplitude modulation output in percent, the output power for the spectrum expansion is undesirably larger. In FIG. 13, the input signal power corresponding to 3 dBm and -3 dBm (dBm should be understood to be a symbol representing the absolute value of decibel with the output power of 1 milliwatt as 0 dBm) are typically shown. In order to satisfy the specification of the frequency spectrum, etc. of the portable phone, the parameters including the amplitude modulation output in percent must be less than a predetermined operating limit within the operating range of transmission power (15 dBm to 35 dBm in the case of FIG. 13). The inventors, however, have found that the specification fails to be met in the hatched portion of the diagram. According to FIG. 13, in the case where the input signal power is fixed to 3 dBm with a comparatively small output power (with a small gate bias voltage and a small power amplification factor of the MOSFET), the waveform was found to be distorted. Nevertheless, an attempt to improve the amplitude modulation output in percent by unduly reducing the input signal power of the radio-frequency power amplifier to, say, -3 dBm as shown in FIG. 13 would fail to produce output power of a magnitude required from the critical gain of the radio-frequency power amplifier.
As explained above, in a modulation scheme using a small to large transmission power and having a transmission signal requiring the amplification linearity, a configuration in which the gain of the radio-frequency power amplifier is changed to produce a desired output power by fixing the input signal power generally fails to secure a sufficient linearity for the power amplification system as a whole due to the nonlinear characteristic of the semiconductors during a small power output. A technique for controlling the gain of the power amplifier is disclosed in U.S. Pat. No. 5,307,512 issued Apr. 26, 1994 to Mitzlaff. This patent is directed to improve a dynamic range of power amplification.